Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 116

the absorption edge of RR-P3HT. The absorption band is already observed at
1250 nm immediately after the laser excitation and still observed at 1250 nm at
100 ps. This finding shows that delocalized P3HT singlet excitons in crystalline
domains can be selectively observed by selective excitation at 620 nm. As
reported in the literatures [ 35 - 37 ], the exciton diffusion constant can be evaluated
by analyzing the dynamics of singlet-singlet exciton annihilation under different
excitation intensities.
As the excitation intensity increases further, the singlet-singlet exciton anni-
hilation is observed even at 0 ps, because singlet excitons are likely to be gen-
erated in proximity to each other at high concentrations without diffusion. The
threshold intensity is 7 9 10 18 cm -3 for RRa-P3HT films excited at 400 nm,
3 9 10 18 cm -3 for RR-P3HT films excited at 400 nm, and 8 9 10 17 cm -3 for
RR-P3HT films excited at 620 nm. Assuming a 3-dimensional sphere without
taking their anisotropic distribution into consideration, an averaged interaction
radius of two excitons at 0 ps can be estimated to be *3.2 nm for RRa-P3HT
films excited at 400 nm, *4.3 nm for RR-P3HT films excited at 400 nm, and
*6.7 nm for RR-P3HT films excited at 620 nm. The difference in the interaction
radius is indicative of the difference in the exciton delocalization at 0 ps. In other
words, singlet excitons are more delocalized in RR-P3HT crystalline films than in
RRa-P3HT amorphous films and upon the excitation close to the band gap than
upon the excitation above the band gap. These are consistent with the peak
wavelengths of the singlet exciton band as discussed above.
5.5.2 Energy Transfer
In some donor-acceptor blends, the efficient energy transfer from photogenerated
singlet excitons is observed before the charge generation at the interface. Here, we
describe the energy transfer in polymer/polymer blends of RR-P3HT and poly(9,9-
dioctylfluorene-alt-benzothiadiazole) (F8BT), which have a large spectral overlap
between the fluorescence of F8BT and the absorption of RR-P3HT. Figure 5.11 a
shows the transient absorption spectra of RR-P3HT:F8BT blend films excited at
400 nm where 60 % of photons are absorbed by F8BT and the remaining 40 % are
absorbed by RR-P3HT. The transient absorption spectra in a picosecond can be
well reproduced by the sum of the S-S absorption spectrum observed for each
pristine film of F8BT and RR-P3HT, suggesting that the major transient species
are F8BT and RR-P3HT singlet excitons in the time domain of \1 ps. In other
words, the broad absorption at around 1000 nm observed at 0 ps is ascribed to
F8BT singlet excitons. The large absorption at around 1250 nm observed after
1 ps is ascribed to RR-P3HT singlet excitons. On the basis of the spectral simu-
lation, the decay dynamics of each transient species can be obtained as shown in
Fig. 5.11 b. The F8BT singlet exciton decays monoexponentially with a lifetime of
*0.3 ps. On the other hand, *40 % of RR-P3HT singlet excitons are promptly
generated immediately after the laser excitation and the other *60 % of RR-P3HT